1. Field of the Invention
This invention relates to metering valves in general and more specifically to 2-way servovalves used for the precise control of the flow of liquids and gasses.
2. Description of Prior Art
Precisely variable control of flow in response to an electrical signal is useful for a number of applications such as welding gas flow control, biomedical gas flow control, mixing of liquids in a laboratory, various functions in automotive fuel and suspension systems, etc. Servovalves use a metering valve in conjunction with an electromechanical actuator to provide this precisely variable control of flow. They are often used in conjunction with electronic feedback of a process variable to provide improved flow regulation. There are a number of advantages to using a rotary valve type to accomplish the metering valve function, including the ready availability of rotary actuation devices such as step motors and torque motors, the elimination of backlash from rotary to linear conversion mechanisms, high frequency response, ease in sealing, and lower cost. Considering rotary metering valve mechanisms, there is a large body of prior art associated with various types of 2-way valves such as ball valves, rotary plug valves, needle valves, and butterfly valves to consider. These devices, in general, fail to provide one or more attributes which are desirable for servovalve applications, including a continuous linear relationship between the input signal and the flow (also known as a linear flow gain), low actuation forces to minimize mechanical hysteresis, low internal leakage when in the closed position, and fast response. In the following discussion, the words xe2x80x9cplugxe2x80x9d and xe2x80x9cspoolxe2x80x9d will be used interchangeably to describe the cylindrical part that is rotated inside the plug-type rotary valve to regulate the flow.
Various approaches have been described to improve one or another of the aforementioned desirable characteristics in the basic 2-way valve types. For example, U.S. Pat. No. 3,4443,793 to Hulsey discloses a rotary plug valve and a similar ball valve, both with slots of complex geometry intended to provide improved metering by varying the orifice flow area of the device as it rotates. The device shown would not, however, have suitable low actuation torque due to excessive seal drag and pressure imbalances inherent in the design. Hulsey discloses another variation to hollow plug and ball type valves in U.S. Pat. No. 3,612,102 in which spiral flow ports in the plug provide the metering function. In addition to the pressure imbalance, the hollow plug disclosed would have substantial axial pressure forces to be restrained by the valve ends, causing additional frictional forces and rendering it unsuitable for servo applications. For plug-type valves in general, this open-end configuration and the resulting axial pressure loads, along with substantial side loads due to imbalanced pressure at the metering port, are common failings in the prior art. For example, an early use of a hollow cylindrical spool in a rotary valve, along with shaped ports to provide linear flow gain is in U.S. Pat. No. 1,141,276 by Smith for a Carburetor. Another example with both pressure imbalances is U.S. Pat. No. 3,342,449 by Jackson.
U.S. Pat. No. 5,242,150 to Shiffler and Loy discloses a rotary servovalve intended to solve the problem of an acoustically quiet valve. In this design, a hollow spool is shown which is closed on both ends, alleviating the end loading problem, although the device is impossible to produce as shown since no method of producing such a central cavity in the otherwise solid spool is described. The method used to feed the inlet flow into the spool requires multiple diameters, greatly complicating the process of producing the spool. The required close fit between the spool and the bore to minimize internal leakage is mitigated against by a multiplicity of parts that must be concentric in order to allow such a close fit. Thrust bearings are described to retain the position of the spool, increasing friction O-ring seals on the spool also contribute to excessive friction. A linearization method for the flow gain is also described, but this requires machining a complex shape in the metering passage in the spool which cannot be accomplished with standard machine tools, and is thus expensive.
Note that all of the prior art discussed above involves cutting complex shapes into the spool and/or its mating receiver part so as to attain the linear flow output with rotation. These shapes are difficult to machine with conventional machine tools and thus add considerably to the cost of the devices.
U.S. Pat. No. 5,868,165 to Tranovich (one of the co-inventors in the current application) discloses a 4-way rotary servovalve in which rectangular slots are opened to a hole to provide linear output flow gain. However, this valve is much more complex than the present invention, is intended for 4-way operation, and utilizes a metering pin which occludes with the metering port in the closed position, and utilizes bearings to fix the position of the spool. The method of feeding the inlet flow to the spool along the outside surface thereof is completely different than that described herein. All of these features drive up the cost and render the valve unsuitable for the 2-way servovalve application.
The first object of the present invention is to provide a valve with the necessary and desirable performance characteristics for servo applications including: 1) minimizing pressure loading and its attendant frictional forces so as to reduce mechanical hysteresis and 2) providing a simple flow path which varies in area, and thus flow, linearly as the spool rotates. It is a further object of the invention to minimize the internal leakage of the valve in its shut off condition. It is another object of the present invention to provide a simple and producible design for a 2-way servovalve, which is easily produced on standard machine tools. The resulting valve mechanism can be easily controlled by rotation with a conventional step motor using well-known micro-stepping technology or preferably with a limited angle servo motor system, such as is described in U.S. Pat. No. 6,034,499 by Tranovich Further objects and advantages will become apparent from consideration of the drawings and ensuing description